1. Field of the Invention
The present invention relates generally to an information-bearing disk. More particularly, the invention relates to the arrangement of data recorded on the disk. The disk may be magnetic (rigid or flexible), magneto-optical (Kerr or Faraday), optical, or an equivalent thereof.
2. Description Relative to the Prior Art
An information-bearing disk includes a plurality of relatively narrow, data-storing tracks encircling the center of the disk. Commonly, the tracks are either circular or in an equivalent spiral form, and are equally spaced from each other by a guardband.
Data is commonly written on the disk at a uniform rate. Thus, when the disk rotates at a constant angular velocity (CAV), the same number of bits occur in each track. Because the length of each track depends on its radius, data density decreases with increasing radius, and only the innermost track may have the maximum allowable bit density. This is very inefficient in terms of data storage capacity.
To avoid the aforementioned inefficiency of the CAV technique, a maximum allowable bit density may occur in each track when the disk is rotated on the basis of a constant linear velocity (CLV) technique. With data written at a constant rate, the angular velocity of the disk must vary in inverse proportion to track radius. For a series of circular tracks, the angular velocity changes in a stepwise manner with each new track. For a spiral track, the angular velocity changes continuously with angular position. Although the CLV technique may maximize storage capacity, it adds significantly to the complexity and cost of a drive mechanism for the disk.
An alternative approach, also known in the prior art, combines features of the CAV and CLV techniques. To that end, the recording surface of a disk is divided into a number of non-overlapping, annular zones of equal width. A track within a given zone has the same number of bits as other tracks in the same zone. For maximum storage capacity for a disk having data arranged in this manner, the innermost track of each zone has the maximum bit density allowed by the system.
While tracing tracks within a zone, data density decreases with increasing radius (as with the CAV system). When the first track of an adjacent zone is reached, there is a step change in rotational velocity (similar to the CLV system), to maintain the required data density during a write operation and a uniform data rate during a read operation. U.S. Pat. No. 4,530,018 discloses a drive mechanism for a disk having equal-width zones.
When operating at a constant data rate, it is required that the linear velocity of the innermost track of each zone be the same. Thus, to retrieve data from any zone at a fixed rate requires that the drive mechanism servo to the appropriate angular speed during a zone accessing interval. It will be appreciated that a high-performance channel decoder for use with disk drive apparatus has a narrow range of operation, for example one percent of a desired data rate; thus readout cannot reliably take place until drive mechanism transients have decayed sufficiently. This places stringent demands on the zone access mechanism and drive motor torque/power requirements.
An alternative arrangement, which consumes significantly less power, is to hold disk angular velocity constant during a read operation and let the data rate vary as data is read from one zone to the next. With reference to the use of a prior art "zoned" disk in this manner, reference is made to FIG. 1, of the accompanying drawings, which is a matrix showing the permutations in read data rate (normalized) for a 5-zone disk, as a joint function of the angular velocity of the disk during a write operation and the angular velocity of the disk during a read operation when the disk is rotated at any one of the speeds used during writing.
With the above-mentioned 5-zone disk, data written at a uniform rate requires that each of zones one through five have angular velocities of a, b, c, d and e, respectively. Assuming that zone 1 is the inner zone and zone 5 is the outer zone, the angular velocity "a", for inner zone 1, would, of course, be the fastest, and velocity "e", for outer zone 5, would be the slowest.
Vertical movement in the matrix of FIG. 1 represents accessing a zone without a corresponding change in disk velocity; horizontal movement represents a change in disk velocity without a change in zone. Thus, each column of the matrix of FIG. 1 shows actual read data rate when only one of these five velocities is employed as a particular read angular velocity.
Note that each of the five main diagonal terms of FIG. 1 has a normalized read data rate of unity. Referring to the two diagonals immediately adjacent the "unitary" diagonal, one might expect that a matrix entry from either diagonal would be approximately equal to the other entries in the same diagonal. In other words, one might expect the term b/a to be close to c/b, and that c/b would approximate d/c, etc., and, in the other diagonal, a/b to equal b/c, etc. Similar logic may be applied to the other diagonals.
With a disk having equal-width zones, however, the terms in a given diagonal, other than the main diagonal, are different from each other.
The matrix of FIG. 1 shows, for a 5-zone disk, that there are 21 different read data rates when any one of the write angular velocities is used throughout a read operation. For the general case of a disk having N equal-width zones, the total number of read data rates equals N.sup.2 -(N-1). The identical terms in the main diagonal explain the existence of the (N-1) term in the general expression for number of read data rates.
It is not possible for a single high performance decoder channel to accommodate the variations in read data rates when angular velocity is constant. Multiple decoder channels must handle the different data rates when only one angular velocity is employed in a given read operation. This, of course, contributes significantly to the cost and complexity of signal processing circuitry for a disk having equal-width zones.